1. Field of the Invention
The present invention relates to a printing apparatus and a printing method in which a print medium is printed by allowing a print head to eject droplets onto the print medium, and in particular, to a printing apparatus and a printing method in which the status of ejection of droplets through ejection ports is sensed.
2. Description of the Related Art
In recent years, ink jet printing apparatuses have been prevailing rapidly in which print media are printed by allowing a print head to eject ink droplets. Many ink jet printing apparatus adopt an ink droplet ejection method in which ink is heated to cause film boiling so that the resulting pressure allows ink droplets to fly. Unlike an electrophotographic scheme, this method requires no intermediate transfer unit and thus needs few intervening elements in forming images. Hence, the method has the advantage of allowing intended images to be stably obtained.
However, in the ink jet printing apparatus, inappropriate ink ejections may result from, for example, blockage of ejection ports by dust or thickened ink, disconnection of wire extending to a heater for the purpose of heating ink, coverage of ejection ports with ink droplets. Such an inappropriate ink ejection may be an ejection failure state in which no ink droplets are ejected through ejection ports, a state in which ink droplets are ejected but in which an ejection amount is smaller than a predetermined value, or deviation of impact positions of ejected ink droplets. When ink is inappropriately ejected, ink droplets may impact the print medium at positions deviating from the desired ones or fail to impact the print medium. This may lead to white or black stripes on print images.
A technique using a light emission element and a light reception element is known as means for sensing the occurrence of inappropriate ink ejection through such ejection ports. In a method for sensing the inappropriate ink ejection state by the sensing means that uses the light emission element and the light reception element, the light emission element and the light reception element are positioned such that ink droplets pass between the elements. Thus, when ink is ejected, a change in the quantity of light passing between the elements is sensed. In this manner, the occurrence of the inappropriate ejection state is sensed by determining whether or not light emitted by the light emission element is blocked by ink droplets.
If the inappropriate ejection state is sensed in the ink jet printing apparatus with the sensing means described above, what is called inappropriate-ejection compensation printing can be performed in which a dot supposed to be formed by an ejection port in the inappropriate ejection state is compensatorily formed by a different ejection port. Thus, even if any ejection port is determined to be in the inappropriate ejection state, the ejection through the ejection port is compensated for to allow formation of high-quality images to be maintained.
The inappropriate ejection state of each ejection port can be found earlier by increasing the frequency of the operation of sensing the inappropriate ejection state. Then, ejection ports in the inappropriate ejection state can be immediately compensated for. Thus, in the conventional printing apparatuses, safety ratio is set to be high, and the frequency of the operation of detecting the inappropriate ejection tends to be set to a relatively large value.
However, in general, the printing operation is suspended while the operation of detecting the inappropriate ejection state of each ejection port is being performed. Thus, when the sensing operation is frequently performed, the time for which printing is suspended increases. If the frequency of the operation of sensing the inappropriate ejection state is high excessively, the time for which printing is suspended increases more than necessary. This may reduce printing throughput.
The recent tendency is such that the length of the print head is increased, with ejection ports densely arranged therein. This tends to further increase the time required for a single operation of detecting inappropriate ejection. Furthermore, the recent year, the application of the printing apparatus has expanded even to large-sized print media for industrial or business use by using of the longer print head. There has been a demand for a further increase in print speed and further improvement of image quality even for printing apparatuses adapted for such large-sized print media. Thus, attempts have been made to further increase the length of the print head (for example, by at least 1 inch) and the integration degree thereof, resulting in an increase in the number of ejection ports in the print head. In such a printing apparatus, when the operation of detecting inappropriate ejection is performed on all the ejection ports during a single process, the operation of detecting the inappropriate ejection state of each ejection port requires much more time.
In contrast, Japanese Patent Laid-Open No. 2007-290352 discloses a printing apparatus configured to perform the operation of detecting ejection ports in the inappropriate ejection state at a frequency corresponding to the accumulated number of ejections in order to suppress a decrease in the throughput of the printing operation associated with the operation of detecting ejection ports in the inappropriate ejection state. This prevents the operation of detecting ejection ports in the inappropriate ejection state from being unnecessarily frequently performed. As a result, a decrease in print throughput can be suppressed. However, the printing is also suspended when the operation of detecting the inappropriate ejection state of each ejection port is performed in the printing apparatus according to Japanese Patent Laid-Open No. 2007-290352. Thus, performing the operation of detecting the inappropriate ejection state of each ejection port increases the time required for printing, resulting in a decrease in printing throughput.
Furthermore, Japanese Patent No. 3382526 discloses a printing apparatus configured to perform printing with the print head carrying out reciprocating scans, wherein during the forward scan, both the printing operation and the operation of detecting the inappropriate ejection state are executed. In this printing apparatus, ejection ports determined to be in the inappropriate ejection state are subjected to compensatory printing during the backward scan. This allows prevention of printing with droplets through which any ejection ports in the inappropriate ejection state in the print head. Thus, degradation of the quality of images obtained by printing can be prevented. However, in the printing apparatus according to Japanese Patent No. 3382526, even when printing is to be performed both during the forward scan and the backward scan, one of the scans needs to be assigned to the operation of detecting the inappropriate ejection state without performing printing. Hence, if printing is performed by a printing apparatus in which the printing is suspended during the operation of detecting the inappropriate ejection state, then during one of the forward and backward scans, the printing is suspended while the inappropriate ejection state detection operation is being performed. This increases the time required for printing, thus reducing throughput.
It is thus difficult to meet both of the conflicting requirements, that is, suppression of a decrease in throughput and improvement of image quality.